1. Field of the Invention
The instant invention is directed to laser machining of metallized substrates. In particular, thin metallized coatings are selectively removed by means of a laser induced explosion.
2. Description of the Prior Art
Additive type processes are known for forming circuit patterns on printed wiring boards wherein a thin film of metal is deposited on the full surface of a substrate and then selectively removed to form the desired pattern. The remaining thin metal film provides a conductive base for depositing additional metal thereon to form a good electrical conductor.
The selective removal step may be accomplished by masking portions of the metallic film with photoresist and chemically etching away the unwanted metallic film. However, it is economically desirable to eliminate these photoresist coating and chemical etching steps.
It is well known to use a coherent beam of light energy from a laser to selectively remove the metallized film from insulative substrates, for example, see U.S. Pat. 3,911,444 to Lou et al. which issued on Oct. 7, 1975. The laser beam scans the metallized substrate and is modulated to directly vaporize predetermined portions of the metallic coating. The remaining metal defines an electrical connecting pattern for printed wiring boards or the like.
With such direct vaporization, the minimum optical energy required for the removal of a particular metallic film, i.e., the threshold value for laser machining, is dependent not only upon the optical properties but also on the thermal properties such as the evaporation temperature and the latent heat of vaporization of the metallic film. Due to the extremely high vaporization temperatures of metals such as copper and gold, which usually form the metallic films, a direct vaporization mechanism requires relatively high laser machining thresholds which undesirably damage heat-sensitive substrates.
Accordingly, the use of a laser to remove thin metallic films from dielectric substrates has not found favor, for the laser evaporation of the metallic films requires substantial laser power and results in unacceptable damage to the substrate.